A well developed technology exists for the purpose of hydrogen formation by means of the electrolysis of water, which is a useful and commercially viable approach investigated throughout the World.
The electrolyzers used therefor are generally constructed in the form of bi-polar units. The working potential of such cells generally speaking lies in the area of 1.8 to 2 volts having a relatively low current density of between 200 to 300 mA/cm.sup.2. The efficiency of such cells is therefore very poor calculated on the reaction enthalpy. For achievement of a 100% yield at economically feasible current densities the total cell potential should be of the order of 1.5 volts. The achievement of this goal is the stated purpose of all commercially viable developments in the field of alkaline water electrolysis.
The greatest problems standing in the way of the achievement of this goal are as follows:
1. An excessively low working temperature for the water electrolyzer. This temperature requirement is determined by the use of the generally employed asbestos diaphragm which is only stable at low temperatures.
2. An insufficient activity of the electrodes and,
3. An unsatisfactory cell construction mode which does not employ the advantages of individual, newly developed, cell components (i.e. active electrodes, diaphrams and the like) to an optimal extent.
Heretofore two principal types of electrolysis cells were known. The most frequently utilized cell construction is based on the concept of Zdansky Lobza (see Lurgi Express Information T1084/6.73) which comprises a cell of the following construction elements:
There are provided two electrodes of nickel netting or perforated sheet placed in a sandwich like manner around a central asbestos diaphragm. This sandwich is placed between a pair of bi-poplar plates in the form of a waffle sheet metal plates whose egg shaped protrusions stand in pressure contact with the electrodes. The sandwich like construction of the electrodes minimizes the electrode distances so that the ohmic potential drop in the electroyltes can be substantially eliminated.
This substantially optimal cell concept does not however, meet the criteria of points 1 and 2 above that is to say, that the net electrodes do not possess a sufficiently satisfactory electro-chemical activity and the asbestos diaphragms prohibit higher working temperatures.
Practically all of the known suggestions for raising the electrode activity are based upon an increase in the surface area of the electrodes by means of a catalytically active porous layer having an increased internal surface area. When such porous electrodes prepared in accordance with the above described construction principles, are combined with bi-polar plates through pressure contact, oxide layer formation particularly on the anode side, gives rise to such high contact resistance that the gains achieved by activation are lost by reduction of the potential through ohmic potential drop at these contact points.
On the other hand, another form of cell construction which comprises an active electrode layer on the bi-polar plates themselves in order to avoid these losses, gives rise to new ohmic potential drops in the electrolyte space between the electrodes and the diaphragm which is filled by the gas produced. This serious problem has been well described in the work entitled "Improvements in Electrolysis Technology in Alkaline Solutions" by Appleby and Crepy (Proceedings of the Second World Hydrogen Energy Conference, Zurich 1978, pages 227-240 in particular section 5).
In this case the problem may be avoided by the introduction of the so called pre-electrodes which are formed out of electro-catalytically surfaced perforated metal, for example, expanded metal (see with Krebskosmo Information No. 5-Water Electrolysis-FIG. 4 and text). There pre-electrodes made of expanded metal, are rigidly connected to the bi-polar plates by spot welding so that no contact losses can occur. However, even in this case there are substantial potential losses caused by solution resistance.
A complete solution to both problems would be offered by an activated porous electrode which is rigidly connected, without pressure contact with the bi-polar plate said electrodes lying sandwich like upon the separator without any space there between. Heretofore this idea has not been technically realized since no simple and commercially viable made of reducing this theory to practice had yet been found.